Here in northern New England, the color comes on slowly, almost furtively. The first leaves turn — on diseased and dying trees — in late July, unbelievably, amid the suffocating lushness of high summer. By mid-August the early sumacs and swamp maples have joined in, and the slide is on despite the heat. Autumn‘s rapidly shortening daylight hastens the change, until one day — and this is my definition of “peak foliage” — you’ll be walking beneath a transcendently bright canopy or driving along an impossibly vivid hillside, and the light that only recently was reflected by the leaves appears, suddenly, to be generated by the leaves themselves. It happens every year, and every year it seems awesome to me, miraculous, even though I’ve lived here my whole life.

I wrote an article once about the chemistry of fall foliage, and learned that the yellow, gold, and orange carotenoids present in leaves remain more or less constant from year to year. Their amounts — and therefore the color they create in autumn when the dominant green chlorophyll shuts down — aren’t affected by temperature or weather. But the anthocyanins — ah, the reds — vary depending on sunlight, moisture, and temperature, making some foliage seasons brighter and more dramatic than others. Longtime Yankee author Ben Rice, more poet than scientist, once wrote that unusually rich autumns are when “the red gods call.”

Among the species rich in anthocyanins — whether displayed all on their own or mixed with the carotenoids — is the sugar maple, the official state tree of Vermont and the unofficial symbol of fall calendars everywhere. Sugar maples are a big part of New England’s mixed northern hardwood forests, and their range is fairly limited. Their numbers dwindle as they move southward into warmer climatic zones; by Connecticut they’ve given way almost completely to the oak and hickory forests that also distinguish the southern Appalachians. With such sensitivity to temperature, the sugar maple has recently emerged as a new kind of poster child: one that warns of how global warming may affect New England. Some models predict that within a couple of centuries, the climate of Vermont and New Hampshire will be more like Virginia’s today — and gone will be maple sugaring and the signature colors of the New England autumn. Such predictions add poignancy to an already bittersweet season.

Since 1991, scientists in Vermont have been intensively studying the health of sugar maples at a site near Mount Mansfield. A series of warm summers and falls have coincided with “abnormally late” color during many of those seasons (up to two weeks later, in some cases, than the average peak, October 3-9). But the scientists have too little data to predict long-term trends or even to blame a warming climate. They’re also trying to measure the impact of disease, insects, acid rain, mercury, drought, extreme cold in low-snow winters, and other stressors — some of which come from the thousands of fossil-fuel-burning vehicles that migrate through the region each fall to linger amidst all that color.

Taking one long view, Tim Perkins, director of the Proctor Maple Research Center, a field station of the University of Vermont, reminded me that the health of the sugar maple is far more vigorous now than it was, say, in the late 1800s, when 75 percent of the state had been cleared. State forester Sandra Wilmot left me with this thought: “Sugar maples,” she said, “are probably the hardiest tree we work with. They adapt to all sorts of insults.”

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